The present invention concerns a tire with radial carcass reinforcement anchored on both sides to at least one bead wire and having a crown reinforcement consisting of at least two superimposed working plies composed of parallel cords or cables in each ply and crossing from one ply to the next at angles with respect to the circumferential direction of the tire of at most 45xc2x0 in absolute value.
More particularly it concerns a xe2x80x9cHeavy Dutyxe2x80x9d type of tire, whose ratio of height at rim H to maximum axial width S is equal to at most 0.80, and is intended to be fitted to medium to high tonnage vehicles, such as lorries, buses, trailers, etc.
Some present-day so-termed xe2x80x9chighwayxe2x80x9d tires are designed to run at high speeds and over increasingly long distances, due to improved road networks and the world-wide growth in motorway networks. The overall conditions under which such a tire is called upon to operate undoubtedly allows a greater number of kilometres to be covered, since the tire wear is less severe; on the other hand, the fatigue strength of the tire and of the crown reinforcement in particular are taxed more severely.
Patent FR 2 419 185 makes it clear that the type of tire in question, with a low H/S ratio, while offering numerous advantages, also has a number of drawbacks, such as poor adhesion of the equatorial area of the tread or a concentration of high pressures in the area of the edges of the tread, due to reduced contact area in the longitudinal direction of the tire. In order to overcome the said drawbacks, the above-mentioned French patent recommends the provision between the carcass reinforcement and the radially internal working ply, in two areas remote from the equatorial plane, of two limiting blocks each composed of two superimposed plies of inextensible cables, crossing from one ply to the next at opposing angles with respect to the circumferential direction, the said angles being equal in absolute value to at most one-half of the smallest angle used in the working plies and above 0xc2x0.
The implementation of the solution outlined above, its application to a tire with a shape ratio H/S equal to at most 0.80, in particular equal to at most 0.6, and intended to be fitted to a xe2x80x9cheavy goods vehiclexe2x80x9d, does not allow a crown reinforcement with a satisfactory fatigue strength to be obtained.
The lack of fatigue strength relates to both the fatigue strength of the crown plies, and in particular the resistance to separation between the edges of the plies, and the fatigue strength of the cables in the part of the carcass reinforcement situated below the crown reinforcement, the former deficiency being heavily influenced by the operating temperatures prevailing at the margins of the working plies, whether the tire is running in a straight line or cornering.
The objective of French patent application 94/015736 is to improve the fatigue strength of the crown reinforcement of a xe2x80x9cheavy goods vehiclexe2x80x9d tire having a shape ratio equal to at least 0.60. the said reinforcement comprising at least two working crown plies made from inextensible cables, crossing from one ply to the next at angles of between 10xc2x0 and 45xc2x0 to the circumferential direction, the said plies having widths equal to at least 80% of the maximum axial width S0 of the carcass reinforcement. It proposes to make provision, firstly, between the carcass reinforcement and the working ply radially closest to the axis of rotation, for an axially continuous ply formed from inextensible metallic cables making an angle of at least 60xc2x0 with respect to the circumferential direction, and whose axial width is equal to at least the axial width of the shortest working crown ply, and secondly between the two working crown plies an additional ply formed from metallic elements and arranged essentially parallel to the circumferential direction, the axial width of the said ply being equal to at least 0.7 S0, and its modulus of elasticity in tension being at most equal to the modulus of elasticity of the most extensible working ply.
French patent application FR 96/02178, which aims to lower the operating temperature of a xe2x80x9cheavy goods vehiclexe2x80x9d type of tire with radial carcass reinforcement, while offering a solution that is at once effective and economical, recommends adding to the said carcass reinforcement a crown reinforcement comprising at least two working crown plies made from inextensible cables, crossing from one ply to the next at angles of between 10xc2x0 and 45xc2x0 to the circumferential direction, and comprising in addition, in the absence of any ply formed from inextensible cables at an angle greater than 45xc2x0 to the circumferential direction, an axially continuous additional ply formed from metal elements arranged essentially parallel to the circumferential direction and positioned radially between the working plies, the said additional ply having an axial width equal to at least 1.05 times the axial width of the widest working crown ply.
While the problems of separation between the working plies and the fatigue strength of the cables in the carcass reinforcement appear to be resolved in one case, and while the operating temperatures are greatly reduced in the other, the long haulage distances covered by tires constructed in this way lead to the appearance of fatigue fractures in the cables of the additional ply and more especially at the edges of the said ply, whether or not the so-termed triangulation ply is present.
It is always possible to change the reinforcing elements, and in particular to choose cables of a different construction or cables with a higher tensile strength. The above solution, while certainly simple, is always costly.
To overcome the above drawbacks and to improve the fatigue strength of the crown reinforcement of the type of tire in question without raising its cost, the tire with radial carcass reinforcement having a maximum axial width S0, according to the invention, comprising a crown reinforcement composed of at least two working crown plies of inextensible reinforcing elements crossing, from one ply to the next at angles of between 10xc2x0 and 45xc2x0 to the circumferential direction, the said plies having axial widths equal to at least 80% of the width S0 and, arranged radially between the said working plies, an additional ply of reinforcing elements essentially parallel to the circumferential direction, is characterized by the fact that the working plies of widths greater than the width of the additional ply by at least 16% of the width S0 are, on both sides of the equatorial plane and in the immediate axial extension of the additional ply, coupled over an axial distance equal to at least 3.5% of width S0, thereafter being separated by sections of a rubber mix over at least the remainder of the width common to the said working plies.
The thickness of the separating profiles between the plies, measured opposite the ends of the narrowest working ply, are equal to at least 2 mm and preferably greater than 2.5 mm.
The term xe2x80x98coupled pliesxe2x80x99 should be understood to mean plies whose respective reinforcing elements are separated radially by at most 1.5 mm, the said thickness of rubber being measured radially between respectively the upper and lower generatrices of the said reinforcing elements.
The term xe2x80x98inextensible elementxe2x80x99 should be understood to mean an element, cable or monofilament, that has a relative elongation of less than 0.2% when subjected to a tensile force equal to 10% of the breaking load. In the case of the tire considered, the inextensible reinforcing elements are preferably inextensible steel cables.
Reinforcing elements, cords or cables, arranged essentially parallel to the circumferential direction, are elements that form angles with the said direction in the range +2.5xc2x0 to xe2x88x922.5xc2x0 with respect to 0xc2x0.
The additional ply advantageously has a modulus of elasticity in tension at most equal to the modulus of elasticity in tension of the most extensible working ply. A modulus of elasticity in tension of a ply of cables results from the tensile stress exerted along the direction of the cables, necessary to obtain a given relative elongation xcex5, the said modulus being a tangential modulus. The expression modulus of elasticity of the additional ply at most equal to the modulus of elasticity of the most extensible working ply""should be understood to mean the tangential modulus of the additional ply, irrespective of the relative elongation, the most extensible ply being the ply which, for any value of the tensile stress, exhibits a relative elongation greater than that of the other ply under the same stress.
Advantageously and in order to facilitate the manufacture of the tire, the modulus of the additional ply will be such that it is low for a relative elongation of between 0% and 0.4%. and at most equal to the highest modulus of elasticity in tension of the most extensible working ply for relative elongations greater than 0.4%.
The working plies are generally of unequal axial widths. The radially outermost working ply can be axially narrower than the radially innermost working ply: it is then advantageous for the crown reinforcement to have an additional radially external ply, known as the protective ply, composed of so-termed elastic reinforcing elements arranged at an angle of between 10xc2x0 and 45xc2x0 to the circumferential direction and having the same orientation as the angle formed by the inextensible elements of the narrowest working ply. The said protective ply can have an axial width smaller than the axial width of the narrowest working ply, and either not cover or totally or partially cover the coupling zone between the two working crown plies. The said protective ply can also have an axial width greater than the axial width of the narrowest working ply, such that it covers the edges of the narrowest working ply and is coupled, in the axial extension of the additional ply, with the widest working crown ply over an axial distance equal to at least 2% of width S0, thereafter being separated axially on the outside from the said widest working ply by sections of a thickness equal to at least 2 mm. The protective ply formed from the elastic reinforcing elements can, in the case described above, be on the one hand possibly separated from the edges of the said narrowest working ply by sections of an appreciably smaller thickness than the thickness of those separating the edges of the two working plies, and on the other hand have an axial width smaller or greater than the axial width of the widest crown ply.
Irrespective of the solution described above, it is advantageous to supplement the crown reinforcement, radially on the inside between the carcass reinforcement and the radially internal working ply closest to the said carcass reinforcement, by a triangulation ply of inextensible reinforcing elements forming an angle with the circumferential direction greater than 60xc2x0 and having the same orientation as the angle formed by the reinforcing elements of the ply radially closest to the carcass reinforcement. The said triangulation ply can have an axial width smaller than the said widest working ply, which is, in the crown reinforcement under consideration, radially closest to the carcass reinforcement. The said triangulation ply can also have an axial width greater than the width of the widest working ply, and it is then advantageous for the radially external so-termed protective ply, coupled to the widest working ply, to be also coupled, in the immediate axial extension of the widest working ply, to the said triangulation ply over an axial distance at least 0.02 times the width S0of the carcass reinforcement, thereafter being separated axially on the outside from the edges of the said triangulation ply by sections having a thickness equal to at least 2 mm. The protective ply formed from the elastic reinforcing elements can then be separated from the edges of the said narrowest working ply, as previously, by sections of appreciably smaller thickness than the thickness of those separating the edges of the two working plies. The said protective ply can also be wider or narrower than the triangulation ply.
The radially outermost working ply can be axially wider than the radially innermost working ply: it is then advantageous for the crown reinforcement to be supplemented internally by a triangulation ply of inextensible reinforcing elements forming an angle with respect to the circumferential direction greater than 60xc2x0 and having the same orientation as the angle of the reinforcing elements of the narrowest ply. The so-called triangulation ply can have an axial width smaller than the axial width of the narrowest working ply, i.e. the ply radially closest to the carcass reinforcement. By preference, the said triangulation ply will have an axial width greater than the width of the narrowest working ply and a width such that it is coupled, in the axial extension of the narrowest working ply, with the widest working ply over an axial distance at least 0.02 times the width S0, thereafter being separated from the edges of the said ply by means of sections of rubber-like mix at least 2 mm thick, regardless of whether it is the triangulation ply or the widest working ply that has the greatest width.
The crown reinforcements just described, with the working ply furthest from the carcass reinforcement being axially the widest, can also be supplemented, radially on the exterior of the said widest working ply, by a protective ply of elastic reinforcing elements arranged at an angle with respect to the circumferential direction of between 10xc2x0 and 45xc2x0 and having the same orientation as the angle of the elements of the radially widest working ply. The said protective ply can have an axial width smaller than the width of the narrowest working ply, and cover, totally or partially, the coupling zone between the two working crown plies. It can also be wider than the narrowest working ply and narrower than the widest working ply, but it will preferably have an axial width such that it radially covers the edges of the widest working ply, being possibly separated from the said edges by sections of a thickness that can be smaller than the thickness of the sections separating the edges of the narrowest working ply from the widest working ply, thereafter being coupled, in the axial extension of the narrowest working ply with the radially internal triangulation ply formed from steeply inclined inextensible elements over an axial distance equal to at least 2% of the width S0, and then separated from the edges of the said triangulation ply by rubber-like sections of a thickness of at least 2 mm, regardless of whether it is the triangulation ply or the protective ply that is the wider.